Heat treatment of aluminum base alloys



Patented July 8, 1941 HEAT TREATMENT OF ALUMINUM ALLOYS BASE Joseph A. Nock, Jr., Tarentum, Pa., assignor to Aluminum Companyot America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

Application July 12, 1939,

Serial No. 283,995

(Cl. 148-Z1.1)

8 Claims.

This invention relates to aluminum base alloys containing zinc, magnesium, copper, and manganese, and is more particularly concerned with the treatment of such alloys to prevent corrosion thereof. By the term aluminum base alloys, I mean alloys containing more than 50 per cent aluminum.

A large number of aluminum base alloys are susceptible to improvement in tensile strength, yield strength, hardness, and other physical properties by various thermal treatments. One of the most common of these treatments, which is usually employed in combination with a subsequent precipitation or age-hardening treatment and is often referred to as a solution heat-treatment, consists in subjecting the alloy to a tem:- perature and for a time sufiicient tocause solution of a substantial quantity of the soluble elements of the alloy but below the fusion temperature of the alloys lowest melting point constituent. These elements then may be retained in solution by a relatively rapid cooling or quenching to room temperature. Following this solution heat-treatment and quenching, the strength, hardness and other properties of some aluminum alloys may be further improved if the alloys are permitted to stand or age for a few days, or are artificially aged at a temperature of about 200 F. to 350F. for a shorter period of time.

Among the alloys which are susceptible to improvement by solution heat-treatment, quenchcially of such articles which are subjected in use i jected to stress, either internal or external in;

source, which tends to fracture the alloy, thereby exposing fresh surfaces to corrosive agents..

Such corrosion in alloys of this type can be substantially prevented if the alloy article is very rapidly or drastically quenched in cold water following solution heat-treatment, but this pro cedure is often impractical because it is apt to produce distortion or warpage of the article.

The object of my invention is to improve theresistance to corrosion of articles of thermally treated aluminum base alloys containing zinc, magnesium, copper, and manganese, and espeto stress. More particularly, the object of the invention is to "provide such a method for-im-' proving the resistance to corrosion of articles of alloys of the class above described, following their solution heat-treatment, which avoids the distortion resulting from the drastic quenching or rapid cooling of such articles. The specific object of my invention is to provide a method for'improvingthe resistance to corrosion of such articles of aluminum basealloys containing-zinc, magnesium, copper, and manganese which have 1 been subjected to solution heat-treatment and have been cooled ata relatively slow rate and subsequently artificially aged. Other. objects ing, and artificial aging are the aluminum base alloys containing zinc, magnesium, copper, and manganese. Such alloys are heated to a temperature between about 850 F. and 1000 F. and maintained at that temperature for a period of from about 5 minutes to about 10 hours in order to place a substantial portion of the soluble constituent of the alloy in solution, and, following quenching to'normal temperature, they are aged usually for about 2 to 30 hours at a temperature between about 200 F. and about 350 F. These alloys develop high tensile and yield strengths and other desirable properties when treated as above described. However, unless they are very rapidly or drastically cooled, following age-hardening they sometimes develop a susceptibility to a type of intergranular corrosion when exposed to corrosive conditions. This attack is believed to be due to the precipitation within the metal of a portionofuthe soluble alloying constituent will appear in the following description of the invention.

My invention is predicated upon the discovery that the resistance to corrosion of solution treated and relatively slowly quenched articles of aluminum base alloys containing zinc, magnesium, copper, and manganese may be increased if they are given a comparativelylow temperature heattreatment following the normal artificial agehardening thereof. This treatment preferably is carried out by heating the articles at the termination of their artificial age-hardening period for a duration of between about 15 minutes and about 2 hours at a temperature of about 350 F. to 450 F. The articles then may be cooled to room temperature in any convenient manner, as, for instance, bynatural cooling or by quenching, without distortion of their shape. This treatment may be practiced at very little expense and is adaptedto be carried out in the heat-treating equipment commonly used.

The above described method has been found to be particularly adapted to aluminum base alloys containing between about 3 per cent and about 10 per cent zinc, between about 0.75 per cent and about 4 per cent magnesium, between about .05 per cent and about 3 per cent copper,

and between about 0.1 per cent and about 1.5 per cent manganese. Various amounts of other elements also may be present to modify or improve certain properties of the alloys, as for example,

alternating immersion oi the specimens into the solution andexposure to the atmosphere was continued ior a period of 48 hours, after which the specimens were tested for the usual physical titanium-y vanadium, zirconium, chromium, co- 5 pmperties,ashereinafter disclosed. belt, tungsten, columbium, or tantalum. The tests showed that the specimens which In a specific application of my invention,1 have had been relatively slowly coole from the sotested specimens of an alloy containing 5.46 per lution heat-treating temperature had suflered cent zinc, 2.21 per cent magnesium, 1.51 per cent much greater losses in properties due to corrosion copper, 0.42 per cent manganese, 0.28 per cent than had those which had been very rapidly iron, 0.17 per cent silicon, and 0.09 per cent tiquenched; and of the slowly cooled specimens, tanium, the remainder of the alloy'being alumithose which had been subjected to=stress during num. These specimens were given a solution exposure to corrosive conditions showed a much heat-treatment often employed for this alloy at greater loss in properties than did the unstressed a temperature of 900 F. for a period of between specimens. These observations indicate that rela- 15 and minutes. Some of the specimens were tively slowly cooled objects of the particular althen very rapidly quenched in cold water andloy have a much lower resistance to corrosion, the others were more slowly cooled by a. high veparticularly it they are subjected to stress while locity air blast. Although all of the specimens exposed to corrosive conditions, than the drastiwere cooled rapidly enough to effect a supersatu- 20 cally quenched objects. rated solution of the alloying constituent in the- Some of the age-hardened specimens which metal at room temperature, the specimens which had been relatively slowly quenched and aged for had been drastically quenched were distorted and 8 hours at 275 F. were then heated for /2 hour twisted by their very rapid cooling, while" the at temperatures of 375, 400, and 425 F. These specimens which had been cooled at a relatively were exposed to the corrosive conditions described slow rate retained their shape. All specimens above, some of the specimens being subjected to were then given an artificial aging treatment for stress during exposure and the others being un- 8 hours at a temperature of 275 F., following stressed. Following 48 hours of alternating exwhich it was found that there was no material posin'e to the salt solution and to the atmosdiiference between the strength and elongation phere, the specimens were tested. The tests of the specimens which had been very rapidly showed that the percentage loss of properties cooled and of those which had been more slowly was not appreciably greater in the slowly cooled from the solution temperature. quenched specimens which had been given a sec- These specimens were then exposed to corrosive ondary relatively low temperature heat-treatment conditions. More specifically, they were repeatthan it had been in the drastically quenched edly subjected to short periods of immersion in specimens, indicating that the resistance to corn n rma 1 11111 m'i q s 801 1 21 11 rosion oi the slowly cooled specimens, even uncontaining 0.3 per cent hydrogen peroxide and der stress, was substantially equivalent to that to short intervening periods of exposure to the of the drastically cooled specimens as a result of atm sp r rin his procedure some of the 40 the above described thermal treatment subsespecimens were subjected to stress equivalent to quent to age-hardening. 75 per-cent oi the yield strength of the alloy The test results are tabulated herewith. The in order that their resistance to stress corrosion numbers preceding the test results are identified might be determined. This repeated cycle of at the end of the'table.

Properties of heat-treated, Percentage loss in properties following quenched, and aged articles exposure to corrosion Unstressed Stressed as; 3%

a mug on s frz h g th Elong' 52 123 1 1251 I Elong Lbalsq. in. Lita/again. Percent Percent Percent Percent Percent 74, 810 64,750 12.8 -4. 43 5 -49 15,900 I 000. 14.0 -10 os -24 -s1 Properties oiheat-treated, quenched,

enthusias Wm I Instressed Stressed E it are. ar a 5 n3 Tensile Tensile strength Elong strength Elong Lba/og. in Lbs/sq. in. Percent Percent Percent Percent Percent 1 Very rapid] quenched from solution heat-treatment temperature.

Relatively owl quenched irom heat-treatment temperature.

Relatively slow y quenched from heat-treatment temperature and treated at 375 F. iollowhg age-hardenin Relatively iollo Relatively 425 F. allowing age-hardening;

g. i owly quenched from heat-treatment temperature and treated at age-hardening. I

owly quenched from heat-treatment temperature and treated at minutes to about 2 hours.

nection with the treatment of a specific alloy, it

may be practiced with advantage to increase the resistance to stress corrosion of articles of other aluminum base alloys containing zinc, magneslum, copper, and manganese which have been cooled from the solution heat-treatment temperature at a relatively slow rate and subsequently age-hardened; and the invention may be utilized with particular advantage where itis desirable to avoid distortion of the shape 01' articles of suchalloys which are .to be subjected to solution heat-treatment and age-hardening.

I claim:

1. A method of heat-treating an aluminum base alloy containing zinc, magnesium, copper, and manganese which comprises heating the alloy at a temperature and for a time sufficient to place a substantial portion of its soluble constituent in solution, quenching the alloy, artificially aging the alloy, and subsequently heating the alloy between 350 F. and 450 F. for from about 15 2. A method of heat-treating an aluminum base alloy containing zinc, magnesium, copper, and manganese which comprises heating the alloy between 850 F. and 1000 F. for from minutes to hours, quenching the alloy at a relatively slow rate, aging the alloy between 225 F. and 350 F.

for a period of between 2 and 30 hours, and subsequently heating the alloy at a temperature of between 350 F. and 450 F. for a period of from about minutes to about 2 hours.

3. A met 0d of heat-treating an aluminum base alloy conta 'ng from about 3 per cent to about 10 per cent zinc, from about 0.75 per cent to about 4 per cent magnesium, from about 0.5 per cent to about 3 per cent copper, and .from about 0.1 per cent to about 1.5 per cent manganese,

which method comprises heating said alloy at a temperature and for a time suflicient to place a substantial portion of the soluble constituent thereof in solution, quenching the alloy at a relatively slow rate, artificially aging the alloy, and subsequently heating the alloy at a temperature of between 350 F. and 450 F. for a period of from about 15 minutes to about 2 hours.

4. A method of heat-treating an article of aluminum base alloy, said alloy containing zinc, magnesium, copper, and manganese, which comprises heating the article at a temperature and for a time sufficient to place a substantial portion of the soluble constituent of said alloy in solution, quenching the article at a rate sumciently 510w to avoid distortion of the shape of the article and sufliciently rapid to retain a substantial portion of the constituent of said alloy in solid solution, artificially aging the article, and subsequently heating the article between 350 F.

and 450 F. for from about 15 minutes to about 2 hours.

5. A method of heat-treating an aluminum base alloy containing zinc, magnesium, copper, and manganese, which comprises heating the alloy at a temperature and for a time sufiicient to place a substantial portion of its soluble constituent in solution, air quenching the alloy, artificially aging the alloy, and subsequently heating the alloy between 350 F. and 450 F. for a period of from about 15 minutes to about 2 hours. I

6. The method of increasing corrosion resistance of an aluminum base alloy containing zinc; magnesium, copper, and manganese, which has been relatively slowly cooled from the solution heat-treatment temperature and artificially agehardened, which method comprises heating said alloy to a temperature ofbetween about 350 F. and about 450 F. and maintaining the alloy at said temperature for a period of about 15 minutes to about 2 hours.

7. In the treatment of an aluminum base alloy containing zinc, magnesium, copper, and manganese to increase its strength and hardness by heating said alloy at a temperature and for a time suificient to place a substantial portion of the soluble constituent thereof in solution, cooling the alloy at a relatively slow rate, and artificially aging the alloy, the subsequent step of heatingsaid alloy for a period of from about 15 minutes to about 2 hours at a temperature of between about 350 F. and 425 F. to improve the resistance of said alloy to corrosion.

8. The method of increasing the strength and hardness of an article of aluminum base alloy containing zinc, magnesium, copper, and manganese, said method. being characterized by a resulting increase in the resistance of the article to corrosion and the absence of substantial distortion to the shape of the article, which method comprises heating said article at a temperature and for a time sufiicient to place a substantial part of the soluble constituent of said alloy in solution, cooling the article at arelatively slow rate, artificially aging the article, and then heating the article at a temperature of between about 350 F. and 450 F. for a period of from about 15 minutes to about 2 hours.

JOSEPH A. NOCK, JR. 

